FR2808891A1 - Liquid crystal display mechanism having liquid crystal material with two right angled stable states and outer electrodes with polariser screen/reflective element and birefringent layer. - Google Patents

Abstract

The bistable display mechanism has a liquid crystal material (30) with two substrates (20,40) with interior face electrodes. The crystal material has two stable positions twisted and non twisted set apart by 90 degrees set by voltage switching. The liquid crystal layer thickness forms a birefringent layer. There is a polariser screen (10) and a reflective element (50).

Description

The present invention relates to the field of liquid crystal display devices.

 STATE OF THE ART According to the physical nature of the liquid crystal used, there are nematic liquid crystal devices, cholesteric, smectic, ferroelectric, etc. In the nematic displays, which are the preferred subject of the present invention, a nematic liquid crystal is used, achiral or chiralized for example by adding a chiral dopant. In this way a uniform or slightly twisted spontaneous texture is obtained, of which no helix is greater than a few micrometers. The anchoring orientation of the liquid crystal near the surfaces is defined by layers or alignment treatments applied to the substrates.

Most of the devices proposed and realized to date are monostable. In the absence of an electric field, only one texture is produced in the device. It corresponds to an absolute minimum of the total energy of the cell. Under field this texture is continuously deformed and its optical properties vary according to the applied voltage. At the cutting of the field, the nematic returns again in the single monostable texture. Those skilled in the art will recognize among these systems the most common modes of operation of twisted nematic (TN), supertordus (STN), electrically controlled birefringence (ECB), vertically aligned nematic (VAN), and the like.

Another class of nematic displays is that of bistable, multistable or metastable nematic. In this case, at least two distinct textures, stable or metastable in the absence of a field, can be produced in the cell, with the same anchoring on the surfaces. Bistables or multistables are generally denominated at least two states of the same energy or very close energy, likely to last almost indefinitely in the absence of external control. On the other hand, metastable states of different energy levels, which can switch after an important time of relaxation, are called metastable. Switching between the two states is achieved by applying appropriate electrical signals. Once the state is entered, it remains memorized in the absence of a field thanks to the bistability. This memory bistable displays is very attractive for many applications. On one side, it allows a low refresh rate of images, very favorable to reduce the consumption of portable devices. On the other hand, for fast applications (eg video) the memory provides a very high rate of multiplexing, allowing a high resolution video display.

Recently, a new bistable display [document 1] has been proposed, using a chiralized nematic liquid crystal or a cholesteric one. The two bistable textures, U (uniform or slightly twisted) and T differ from each other by a twist of 180 and are topologically incompatible (Figure 1). The spontaneous pitch po of the nematic is chosen close to 4 times the thickness <I> d </ I> of the cell <I> (po - 4.d) </ I> to make the energies of U and T substantially equal. . Without a field, there is no other state with a lower energy: U and T have true bistability.

Due to the topological incompatibility of the two bistable textures, it is impossible to transform them into one another by a continuous distortion in volume. Switching between U and T therefore requires anchoring transitions on the surfaces, induced by an external field.

above a threshold electric field E ,, (anchorage breaking threshold), an almost homeotropic texture (referenced H in FIG. 1) is obtained, with at least one of the anchors on the substrates broken: molecules are normal to the blade in the vicinity of this surface.

At the cutting of the field, the nematic molecules near the broken surface are in unstable equilibrium, without any anchoring torque, and can return either to their initial orientation (realizing the same texture as that before the application of the field) or rotate at 180 and induce after relaxation a volume texture with an additional twist of 180. At the end of the control pulse, the cell is guided to one or the other of the bistable states, depending on whether the coupling between the movements of the molecules near the two surfaces is elastic or hydrodynamic: the elastic coupling gives a return to the state U, the hydrodynamic coupling to the state T. For the information displayed on the device to appear, it is necessary that the textures made have different optical properties. Most devices work in polarized light and use additional optical elements: polarizers, filters, compensating plates, and so on. These elements and their orientations with respect to the anchors on the two surfaces are chosen according to the configuration of the display, so as to optimize the relevant optical performance: contrast, brightness, colorimetry, viewing angle, etc.

For monostable displays, the optimization must cover a whole continuum of states, made under more or less strong field, because these states are displayed during the entire duration of an image. A very large number of optical geometries have been proposed and realized for different devices (TN, STN, etc.), taking into account the particularities of each of these displays.

The optics of the bistable display with anchoring break is very different from that of the monostable devices. First, for most of the duration of an image, only two textures are made in each element of the display: those corresponding to the two bistable states. The optimal configuration must allow maximum contrast between these two textures, while minimizing the transient optical effects during switching, due to fast passage through intermediate states in the field. On the other hand, the main difference between the two bistable textures, the additional twist of 180, is not a free parameter for optimization: it is imposed by the physical mechanism of the realization of the two bistable states. In addition, the bistable switching requires a strong electric field E> Ec (close to 10 Vgm) and therefore a control voltage U = E.d which is proportional to the thickness d of the cell. The liquid crystal layer must therefore be very thin (d - 2 - 3 gym) to allow the control by reasonable tensions and the optical optimization must take into account these requirements. The object of the present invention is now to provide a novel liquid crystal display device which has superior properties to prior known devices.

This object is achieved in the context of the present invention, thanks to a bistable display device in reflection, characterized in that it comprises a) a liquid crystal material contained between two parallel substrates, provided with electrodes, their inner surfaces facing each other. to enable the electric field to be applied to said liquid crystal, at least the front substrate and the front electrode being optically transparent, b) alignment layers or treatments on the electrodes which orient the liquid crystal and allow the alternative realization of at least two stable or metastable distinct textures in the absence of a field, where one of the textures is either non-twisted or twisted at a total angle of between -90 + 90, and the other texture has an additional twist of an angle close to 180, c) the thickness d of the liquid crystal layer being chosen such that product d.dn is close to ÂOM, where ΔO is the long the median wavelength of the useful spectral band of the display and dn is the birefringence of the liquid crystal for this wavelength, d) means designed to apply, on the liquid crystal, electrical signals that make it possible to switch by breaking anchoring on at least one of the two substrates, between said distinct textures and remaining in one of them after the removal of the field; e) a polarizer, associated with the front face of the device (placed at the inside or outside thereof) and oriented at an angle between 15 and 75 with respect to the director of the liquid crystal on the front face of the device, and f) a reflective, specular or diffusing element, placed on the rear face of the liquid crystal, inside or outside the device, allowing the light to pass twice in the device and return to the observer or to additional optical elements.

 The reflection bistable display thus proposed according to the present invention offers many advantages.

In particular, it can keep and display an image for a very long time without any energy consumption, neither for operation (it is bistable), nor for its lighting (it does not require an internal light source).

This bistable device in reflection can be optimized taking into account different parameters. With a single polarizer, it allows multiple configurations, which provide a contrast of 50 to 60 in white light. Without loss of optical quality, optimization also makes it possible to use a minimum thickness of the cell, which makes switching faster and reduces the control voltages required for switching.

According to other features of the invention. the liquid crystal material comprises a liquid crystal or a liquid crystal mixture in a nematic phase, the liquid crystal material comprises a liquid crystal or a liquid crystal mixture in a cholesteric or nematic phase doped with chiral substance, to allow to bring closer or equalize the energies of certain textures among the stable or metastable textures,. the optical retardation <I> d An </ I> of the liquid crystal layer is within the limits 0 λ 0 = 0.35-λ 1 .1, preferably 0.20-λ, λ = 0.32.m, where λ .o is the median wavelength of the useful spectral band,. the polarizer is a linear or elliptical polarizer, a compensating blade is introduced into the optical path between the polarizer and the reflector, inside or outside the device, with an optical delay <B> AI </ B> of less than ao / 12 where # 4 is the length median wave of the useful spectral band,. at least one of the electrodes contains several different segments to make it possible to produce several independent pixels (pixels) on the same substrates and in the same device, the independent pixels (pixels) are provided with independent means for applying the field,. the independent pixels (pixels) are organized in a multiplexed passive matrix,. the independent image elements (pixels) are organized in a multiplexed active matrix,. the polarizer is oriented at an angle close to 45 with respect to the liquid crystal director on the front face of the device, the torsion angle of the first texture is practically zero (d0 - 0), the device is optimized, as regards the torsional angle of texture in the low twist state, the relative torsion between the two states, the orientation of the polarizer with respect to the alignment of the liquid crystal the front face, the thickness of the liquid crystal material placed between the two substrates and the birefringence of the liquid crystal so as to obtain optimal optical performance, especially contrast, luminance, colorimetry, the optical axis of the compensating blade is oriented substantially at 45 relative to the polarizer, liquid crystal introduces an optical delay of between 100 nm and 180. the compensating blade introduces an optical delay of less than 50 nm, the polarizer is combined with the compensating blade as a single element to make an elliptical polarizer, the thickness of the liquid crystal material is less than 6 gm.

Other features, objects and advantages of the present invention will appear on reading the detailed description which follows and with reference to the accompanying drawings given by way of non-limiting examples, and in which. Figure 1 previously described schematically shows the three states that can be obtained with a display according to the state of the art,. FIG. 2 schematically represents a perspective view of a device according to the present invention, Figure 3 schematizes the return of the light, thanks to the reflector, in the device according to the present invention,. Figures 4 and 5 diagrammatically different solutions of equations, depending on the orientation of the polarizer, which will be explained later,. FIG. 6 illustrates a particular branch of a solution according to the present invention giving a good contrast in white light, FIG. 7 represents the reflectivity R around the first branch, calculated at d, <I> An </ I> and dOfixes at Wk variable, FIG. 8 represents the reflectivities R (P, WI) of "white" states for a given branch of solution, FIG. 9 shows the reflectivity of two states as a function of the optimal configuration of the device in reflection according to the present Figure 10 presents colorimetric curves, calculated for a standard source D65, as a function of d-An, Figure 11 represents the contrast calculated in white light,. Figure 12 schematically shows in perspective a device according to the present invention incorporating an additional compensating blade,. Figures 13 and 14 show various optimization solutions which will be explained later,. Figure 15 shows the reflectivity of corresponding twisted states, and. Figure 16 shows the reflectivity of two states.

DESCRIPTIVE PART Like the other displays in reflection, the bistable devices can be made in many configurations, with one or two polarizers, with one or more compensating blades, etc. In the present invention the device comprises a single polarizer, placed in the path of light, on the front face of the device. This configuration has the important advantage of maximum brightness because it minimizes light losses due to a possible second polarizer.

The simplest configuration of the device according to the present invention only comprises, as illustrated in FIG. 2, a polarizer 0 on the front face and a mirror 50 on the rear face of the liquid crystal layer 30 placed between two substrates 20, 40 , provided with electrodes on their internal surfaces, without any additional optical element (eg compensating blade).

In FIG. 2, the orientation of the polarizer 10 is referenced 12, the alignment directions respectively defined on each of two substrates 20, 40 are referenced 22 and 42 (ie a torsion angle), and the two states U and T likely to be occupied by the liquid crystal material 30.

To optimize the device according to the present invention, it is possible to play on all the parameters which define the optical properties of the display: <b> the angle of torsion d (Po of the texture in the state U of low torsion (00o I <_180), the additional torsion 180 in the second bistable state T, the orientation P of the polarizer 10 relative to the alignment of the liquid crystal 30 on the front face 20 (-90 <_ P < 90), the thickness d of liquid crystal material 30 placed between the two substrates 20 and 40 and the birefringence An of the liquid crystal.

These parameters will be chosen so as to obtain optimal optical performance of the device, in particular contrast, luminance, colorimetry, etc.

A particularity of the bistable displays is the fact that most of the time only two states are realized and therefore only these two bistable states must be optimized optically.

It will be demonstrated that in general, for any orientation P of the polarizer 10, several solutions will give optimal optical performance. The choice between solutions will also optimize the switching of the liquid crystal, for example by reducing the thickness of the layer 30, without loss of optical quality.

Indeed, the bistable devices require the application of a very strong electric field E, close to 10 V / pm, imposed by the usual very high thresholds of anchor breaks. The control voltages U = d E are therefore large compared to conventional displays. A decrease in the thickness will allow to reduce the same factor.

The optical relaxation times after switching, proportional to d2, are also favorably shortened to low d, which is very important for fast applications, for example for video display.

Finally, the switching of the bistable nematic is controlled by the hydrodynamic shear flows, launched at the end of the control pulse by the weak elastic couplings between the two surface anchors. A low thickness of the liquid crystal strengthens the hydrodynamic and elastic couplings and thus promotes a more efficient control of the display.

Those skilled in the art will understand the importance of the optimal configuration presented in this invention, which improves both the optical quality, speed, control voltage, and switching of the device between the two bistable states.

s <I>a</I> sont des fonctions de la biréfringence intégrée du cristal liquide d-An longueur d'onde de la lumière A., de l'angle de torsion de l'état dO et l'orientation du polariseur P. For a uniformly twisted texture, whose pitch is much larger than the wavelength of light, when the propagation is parallel to the axis of the helix, there are well-known analytical formulas [document 2 ], which describe with a good approximation the optical properties of the system. Taking into account the double passage of light in the device (as shown schematically in Figure 3) and assuming a perfect polarizer 10 and mirror 50, we obtain the following formulas for the reflectivity of the display R = I-cos 2 (s) <B> cos </ B> 2 (2P-a) (1)

s <I> a </ I> are functions of the integrated birefringence of the liquid crystal d-An wavelength of the light A., the torsion angle of the state dO and the orientation of the polarizer P .

Toutes ces solutions correspondent à un contraste infini en lumière monochromatique avec Zo=d.d k. En pratique, on demande à l'afficheur d'assurer un bon contraste aussi en lumière blanche, i.e. quand # varie autour de Àk (à Pet dO fixes) R doit rester proche de zéro. To obtain an optimal contrast in monochromatic light, it is necessary that bistable states, torsion dO or dGD-ter, be black (R = 0). For each orientation of the polarizer 10, equation (1) has a series of solutions (ek, dOk) that provide this condition (Figures 4 and 5). first branches (k = 0,1,2 or 3) solutions MP) and dOk (P) are shown in Figures 4 and 5 where for simplicity it is limited to polarizer orientations <B> 10: </ B> - 45 sPs + 45 and torsion of the cell <0. The solutions with dO> 0 correspond to 45 <_P <_135 and can be obtained on the same figure by the transformation

All these solutions correspond to an infinite contrast in monochromatic light with Zo = dd k. In practice, the display is asked to provide a good contrast also in white light, ie when # varies around Ak (at Pet dO fixed) R must remain close to zero.

The first branch (@o, d0o) shown in FIG. 6 best satisfies this criterion and corresponds to a contrast in white light that is much higher than the other branches. Below we will only consider this solution. Figure 7 shows the reflectivity R around the first branch calculated at d, <I> An </ I> and fixed OO at variable Xolk. For P between -10 and +45 (or between 45 and 100 if dO <0) the window corresponding to a low reflectivity around the branch is wide and almost independent of P. This region corresponds to an optimal optical quality of the black state of the display in white light (low average reflectivity and low X dependency).

It can be seen that in this configuration the optimization of the black state does not impose a particular orientation of the polarizer but only - 10 <_ P <_ + 100. It is therefore possible to use the freedom choice of P to optimize, at equal optical quality, the thickness d of the liquid crystal layer 30, a minimum thickness being favorable for improving the switching.

FIG. 5 shows directly that this minimum thickness d <I> I </ I> (4dn) is obtained for P = 45 and that it varies slowly in the region <P <+75.

This thickness is twice as small as the optimum value in transmission [document 1] or reflection with two polarizers.

The device according to the present invention thus allows, with P near '45, to lower the control voltages by a factor of two, while keeping a good optical quality. The relaxation times after the field cut are divided by four and can become of the order of a millisecond, compatible with the high resolution video display.

For the "white" bistable state of the device, a strong reflectivity, preferably R = 1, and a low wavelength dispersion are required. These two conditions ensure maximum luminance and colorimetry of the white state. The only difference between the black state (already optimized) and the white state (to be optimized) is an additional twist of 180.

FIG. 8 shows the reflectivities R (P, 7dl) of the "white" states corresponding to the zero branch of the already optimized black state. We see that in all cases R is very close to 1 and depends very little on 4 & for +1 <P <+75.

This confirms the interest of the optimal configuration of the reflective bistable display according to the present invention with P-45, d 71, o <I> l </ I> 4-4n (where #, o is chosen close to the median bandwidth of the display) and dO - 0 for the state of low torsion. In. indeed, this configuration optimizes not only the optics of the two bistable states, but also allows the use of a minimum thickness of the liquid crystal layer 30, and therefore improves switching between the two states.

FIG. 9 shows the reflectivity of the two states for P = 45, as a function of .II o for the optimum configuration of the device in reflection. reflectivity of the white state or texture T (dO = 80) varies very slowly around À = Âo. On the other hand, the black state or texture U (do = 0) has a non-negligible dispersion R (,.), which decreases the contrast in white light gives a colored <I> black </ I> state.

Figure 10 shows the color curves, calculated for a standard source D65, as a function of d-An. The light state (dO = 180) is very close to perfect white, but the dark state (dO = 0) is colored.

contrast calculated in white light (Figure 11) optimal (-57) for d-An = 137nm, corresponding to a very dark blue color of the dark state = 0.

According to a variant as illustrated in FIG. 12, further improvement of the reflection bistable display proposed in the present invention comprises a compensator blade 60 of slight optical thickness, introduced between the polarizer 10 and the reflector 50, inside or outside the outside of the device. The optical axis 62 of this uniaxial compensating plate 60 is chosen essentially at 45 relative to the polarizer 10.

The optical path difference introduced by the compensator 60 in a single passage of light through it is where d. is its thickness and dnc, its birefringence (positive or negative). The corresponding angular phase shift is defined by S = 2nd @ dnd, #, where # is the wavelength of light at the center of the spectral band, for which the device needs to be optimized.

Qualitatively, the role of the compensator blade 60 of low phase shift is as follows. It has already been demonstrated that the optimal optical configuration of the device corresponds to an optical thickness of the U-state close to d. An = X014. However, for the switching of the device it is preferable to reduce as much as possible the thickness d of the nematic 30. These two conditions can be satisfied simultaneously by replacing a portion of the phase shift of the liquid crystal 30 by a phase shift introduced by the compensating blade 60. '.An + 8, = Xo / 4 where <Xol (4. is a lower thickness) The black state in this case will retain its optical properties The color and brightness of the white state will be slightly degraded , but this undesirable effect can be overtaken by optimal choice of the parameters P and dO if the phase shift 8, = 2ndAn, /, Z, small (1 radian).

Figures 13 and 14 show the first solution branches and dO (P, b), calculated for R (dO) = 0 and for 8 = -10, 0, and +20.

FIG. 15 shows the reflectivity of the corresponding twisted states R (dO) (similar results are obtained for dO +; z). Those skilled in the art will understand from these figures that the introduction of a small positive phase shift ( 8 - 15) makes it possible to further reduce the thickness of the liquid crystal layer 30.

For a larger phase shift, the light state e <B> n> </ B> @r gradually becomes less bright and strongly colored, which decreases the optical quality of the display.

In figure 16 we present the reflectivities of the two states and dO - ir calculated for dO-- -25,4, P = 30, <B> <I> & - </ I> </ B> 15, Âo = 560nm, eo) = 0 depending on # o /.. This configuration optimized to an optical quality comparable to the non-compensator case 60 (8 = 0), while allowing a thinner liquid crystal layer thickness of about 15% to be used.

In the foregoing calculations, it is assumed that the dispersion of the compensator 60 is similar to that of the liquid crystal itself. In practice, the judicious choice of the compensating dispersion 60 (for example with the opposite sign) may give additional advantages, notably a lower dispersion in the black state of the device and therefore a better contrast in white light.

Embodiment Example: A nonlimiting example of the device proposed in the present invention was produced and studied by the inventors. To demonstrate its advantages it has been compared to a bistable cell by anchoring break optimized for transmission display.

In both cases, one of the anchors on the surfaces 40 is strong and with an inclination angle of about 30 (SiO 2 grading evaporation at 85). On the other surface a weaker planar anchorage was imposed by evaporation of SiO at 75. The commercial nematic 5CB (Merck) was chiralized adding the chiral dopant S 811 (Merck). concentration adopted for each of the two cells was adapted to obtain a spontaneous pitch of the mixture Po = 4.d (d = 1.5 gm for the transmission cell according to the state of the art and d = 0.85 Nm for the cell in reflection according to to the present invention).

The static thresholds of breakage of the anchorage measured on the weak anchoring plate for the two cells were comparable (E = 7 Vlgm). The reproducible switching between the two bistable states U and T was carried out with the same electrical signals for the two devices, but with very different voltages: U = 18 V for the transmission cell according to the state of the art (d = 1.5 gm) and U = 8 V for the reflection cell proposed in the present invention (d = 0.85 gm). The optical relaxation times after switching, measured in the reflection cell according to the present invention (z = 2 ms) are also much lower than those in the transmission cell according to the state of the art (T = 6 ms ). These results confirm the great applied interest represented by the reflection configuration proposed in the present invention.

Of course, the present invention is not limited to the embodiments which have just been described, but extends to all variants that are in keeping with its spirit.

[1] FR-A-2,740,894 [2] Appl.

 Phys.

 Lett. <B> 51 </ B> (18) Nov 1987 Optical properties of general twisted nematic liquid crystal displays, H. L. Ong

Claims (14)

<B> CLAIMS </ B> 1. Bistable display device in reflection characterized in that it comprises a) a liquid crystal material (30) contained between two parallel substrates (20, 40), provided with electrodes on their inner facing surfaces, to allow applying an electric field on said liquid crystal, at least the front substrate (20) and the front electrode being optically transparent, alignment layers or treatments on the electrodes, orienting the liquid crystal and allowing the alternative embodiment at least two distinct stable or metastable textures in the absence of a field, where textures is either undistorted or twisted at a total angle between -90 and +90, and the other texture has an additional twist of an angle close to 180, c) the thickness d of the liquid crystal layer (30) being chosen such that the product d.dn is close to .'i, 014, where .t, 0 is the median wavelength of the spectra band the usefulness of the display and <I> An </ I> is the birefringence of the liquid crystal for this wavelength, means designed to apply, on the liquid crystal, electrical signals which make it possible to switch by breaking the anchoring on at least one of the two substrates, between said distinct textures and remaining in one of them after the removal of the field, a polarizer (10), associated with the front face of the device, placed at the inside thereof, and oriented at an angle between 15C> and <B> 750 </ B> with respect to the director of the liquid crystal on the front face of the device, and f) a member (50) reflective, specular or diffusing, placed on the back side of the liquid crystal, inside or outside the device, allowing the light to pass twice through the device and return to an observer or to additional optical elements . 2. Device according to claim 1, characterized in that the liquid crystal material (30) comprises a liquid crystal or a mixture of liquid crystal in a nematic phase. 3. Device according to claim 1, characterized in that the liquid crystal material (30) comprises a liquid crystal or a mixture of liquid crystal in a cholesteric or nematic phase doped with a chiral substance, to allow closer or equalize the energies of certain textures among stable or metastable textures. 4. Device according to one of claims 1 to 3, characterized in that the optical delay <I> d An </ I> the liquid crystal layer (30) is within 0.15-1.o = 0, 35-Ao, preferentially 0.20e = 0.32e1.0, where 4 is the median wavelength of useful spectral band. 5. Device according to one of claims 1 to 4, characterized in that the polarizer (10) is linear or elliptical polarizer. 6. Device according to one of claims 1 to 5, characterized in that a compensating blade (60) is introduced into the optical path between the polarizer (10) and the "flector (50), inside or out. the outside of the device, with an optical delay A1 less than? 112 where 4 is the median wavelength of the useful spectral band. 7. Device according to one of claims 1 to 6, characterized in that at least one of the electrodes contains several different segments to make it possible to produce several independent pixels (pixels) on the same substrates and in the same device. 8. Device according to one of claims 1 to 7, characterized in that the independent image elements (pixels) are provided with independent means for applying field. 9. Device according to one of claims 1 to 8, characterized in that the independent image elements (pixels) are organized in a multiplexed passive matrix. 10. Device according to one of claims 1 to 8, characterized in that the independent image elements (pixels) are organized in a multiplexed active matrix. 11. Device according to one of claims 1 to 10, characterized in that the polarizer (10) is oriented at an angle close to 45 relative to the director of the liquid crystal front face of the device. 12. Device according to claim 11, characterized in that the torsion angle of the first texture is substantially zero (dO-0). 13. Device according to one of claims 1 to 12, characterized in that it is optimized, as to the torsional angle (doo) of the texture in the state of low torsion, the relative torsion (180) between the two states, the orientation (P) of the polarizer (10) relative to the alignment of the liquid crystal (30) on the front face (20), the thickness (d) of the liquid crystal material (30) placed between the two substrates (20, 40) and the birefringence (An) of the liquid crystal so as to obtain optimal optical performances, especially contrast, luminance, colorimetry. 14. Device according to one of claims 1 to 13, taken in combination with claim 6, characterized in that the optical axis (62) of the compensating blade (60) is oriented substantially at 45 relative to the polarizer ( 10). Device according to one of claims 1 to 14, taken combination with claim 6, characterized in that the liquid crystal (30) introduces an optical delay between 100 nm and 180 nm. Device according to one of claims 1 to 15 taken combination with claim 6, characterized in that the compensating blade (60) introduces an optical delay less than 50nm. Device according to one of claims 1 to 16, taken combination with claim 6, characterized in that the polarizer (10) is combined with the compensating blade (60) as a single element to achieve an elliptical polarizer. 18. Device according to one of claims 1 to 17, characterized in that the thickness of the liquid crystal material (30) is less than 6 pm.